imx: ventana: add mem_mb dynamic env var

Certain OS bootscripts need to know how much memory a board has to adjust
kernel parameters (namely Android). This allows those boards to determine
mem size in MB.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>

board/gateworks/gw_ventana/gw_ventana.c

@@ -1498,6 +1498,10 @@ int misc_init_r(void)
 		/* board serial-number */
 		sprintf(str, "%6d", info->serial);
 		setenv("serial#", str);
+
+		/* memory MB */
+		sprintf(str, "%d", (int) (gd->ram_size >> 20));
+		setenv("mem_mb", str);
 	}